Portable handheld work apparatus such as motor-driven chain saws, brushcutters, suction/blower apparatus or the like have an internal combustion engine as a drive motor. These apparatus have a carburetor arrangement for supplying the engine with an air/fuel mixture. The carburetor arrangement must ensure a good starting performance and also a good acceleration performance of the engine.
An adequately rich air/fuel mixture is required during a sudden acceleration operation. This air/fuel mixture cannot reliably be adjusted exclusively via the suction forces acting on the fuel nozzle in the intake channel. So-called accelerator pumps are known for making available additional fuel quantities when pulling on the throttle. These accelerator pumps are coupled, for example, to the position of the throttle flap. Accelerator pumps of this kind include a piston which pumps an additional quantity of fuel into the intake channel of the carburetor when opening the throttle flap. The internal combustion engine can then be cleanly run up with the short-term enriched air/fuel mixture.
After longer standstill times of the work apparatus, air or fuel vapor bubbles can form in the fuel system and especially in the fuel conducting parts of the carburetor. To avoid the starting difficulties associated therewith, carburetor arrangements are provided with venting pumps by means of which fuel including possibly collected air bubbles can be pumped out of the carburetor and back into the fuel tank so long until the fuel-conducting parts of the carburetor are filled free of bubbles with after-flowing fuel. Venting pumps of this kind are also characterized as purgers.
A flow-conducting connection between the venting pump and the accelerator pump via a venting line is also provided for a complete venting of the carburetor. In the venting line, a check valve is mounted which opens toward the purger. A further check valve is provided downstream of the venting pump. The venting pump includes, for example, an elastic pump bellows. By pressing the pump bellows, the fuel is moved from the interior of the pump bellows through the outflow-end check valve to the tank. Thereafter, the pump bellows returns to its original form because of the elastic material characteristics thereof. Fuel is drawn by suction through the inlet end check valve in the venting line by the accelerator pump.
A fuel pressure builds up in the accelerator pump when there is a sudden actuation of the throttle which pressure is provided for moving fuel into the intake channel. Via the flow-conducting connection of the accelerator pump to the venting pump, a portion of the discharged fuel can flow off through the venting line and the venting pump. The corresponding component portion is then not present for the formation of the mixture. A check valve located between the accelerator pump and the venting pump, which is adequately pretensioned in the closing direction to avoid this effect, can lead to the condition that the pump forces become unwantedly high during the venting operation. The elastic return formability of the pump bellows can then not be adequate. It has been shown that the spring force in the check valve can lead to the situation that the valve remains closed with a slight injection pressure from the accelerator pump; whereas, for a very sudden depression of the throttle causing a high injection pressure, the check valve opens. The result is that a non-predictable undefined fuel quantity becomes lost through the venting line in the direction of the tank. A matching of the carburetor arrangement to the acceleration operation is difficult.